docs: def spiral(Di, rmax, rmin, pitch, fd):
      r'''Returns loss coefficient for any size constant-pitch spiral
      as shown in [1]_. Has applications in immersed coils in tanks.
  
      .. math::
          K = \frac{r_{max} - r_{min}}{p} \left[ f\pi\left(\frac{r_{max}
          +r_{min}}{d}\right) + 0.20 + 4.8f\right]
          + \frac{13.2f}{(r_{min}/d)^2}
  
      Parameters
      ----------
      Di : float
          Inside diameter of pipe, [m]
      rmax : float
          Radius of spiral at extremity, [m]
      rmin : float
          Radius of spiral at end near center, [m]
      pitch : float
          Distance between two subsequent coil centers, [m]
      fd : float
          Darcy friction factor [-]
  
      Returns
      -------
      K : float
          Loss coefficient [-]
  
      Notes
      -----
      Source not compared against others.
  
      Examples
      --------
      >>> spiral(Di=0.01, rmax=.1, rmin=.02, pitch=.01, fd=0.0185)
      7.950918552775473
  
      References
      ----------
      .. [1] Rennels, Donald C., and Hobart M. Hudson. Pipe Flow: A Practical
         and Comprehensive Guide. 1st edition. Hoboken, N.J: Wiley, 2012.
      '''
      return (rmax-rmin)/pitch*(fd*pi*(rmax+rmin)/Di + 0.20 + 4.8*fd) + 13.2*fd/(rmin/Di)**2